This invention is concerned with a process for converting coal and other solid, highly viscous liquid, or gaseous fossil fuels to liquid petroleum products, particularly hydrocarbon fuels. It more especially is concerned with converting such materials to high quality gasoline.
The increasing demand for high octane gasoline has been met, until now, by advanced petroleum refining technology. The two processes which have made it possible to satisfy the demand are: catalytic cracking, which serves to increase the fraction of crude petroleum which can be brought into the gasoline boiling range; and catalytic reforming, which serves to upgrade the octane number of low grade gasoline. Obviously, the cracking process increases the gasoline yield at the expense of the heavier fuel fractions, such as No. 2 fuel oil, which also is subject to growing demand. The increasing cost of petroleum, together with the foreseen increase in demand for gasoline and other petroleum fractions, makes it necessary to seek other fuel sources from which to make high quality gasoline.
Coal, for example, is an obvious alternative raw material, since there are abundant deposits of this fuel. Gasoline has been made from coal by gasification and conversion of the gases to gasoline by the Fischer-Tropsch process. However, this process, not presently used in this country, produces an extremely poor quality of gasoline, with an octane number of about 55, which cannot be efficiently upgraded by the known catalytic reforming processes because it consists predominantly of straight-chain aliphatic hydrocarbons.
Processes for the conversion of coal and other hydrocarbons to a gaseous mixture consisting essentially of hydrogen and carbon monoxide or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Such a gaseous mixture hereinafter will be referred to simply as synthesis gas.
Although various processes may be employed for the gasification, those of major interest for the present invention depend either on the partial combusion of the fuel with oxygen, or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. In one known variant, for example, coal may be completely gasified by first coking, and then subjecting the coke to a cyclic blue water gas process in which the coke bed is alternately blasted with air to increase the bed temperature and then reacted with steam to produce the synthesis gas. The gases generated by the coking step may be used as fuel or steam-reformed to additional synthesis gas. In another process coal or coke may be reacted with highly superheated steam or with oxygen and steam to produce synthesis gas. Regardless of the process variants chosen, oxygen rather than air is often used in the chemical step in which the fuel is converted to synthesis gas since the use of air would result in a gas that contained excessive amounts of inert nitrogen.
An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433, (1966), Interscience Publishers, New York, N.Y., the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, viscous or gaseous fuel are not considered to be per se inventive here.
It is known that raw synthesis gas contains one or more of the following impurities: sulfur compounds, nitrogen compounds, particulate matter and condensibles. The art of removing these contaminants is known, and is described in the above reference and elsewhere. Particular attention is called to the sulfur compounds. It is desirable to reduce this contaminant below a prescribed level for ecological purposes.
Purified synthesis gas ordinarily contains a volume ratio of hydrogen to carbon monoxide plus carbon dioxide of from as little as about 0.10 to as much as 1.1, depending on the particular fuel and process used; in most instances, the composition has a volume ratio from about 0.30 to about 0.65. It is well known that this ratio may be increased by the catalytic carbon monoxide shift reaction described by the equation: EQU CO + H.sub.2 O .revreaction. CO.sub.2 + H.sub.2
with subsequent removal of at least part of the produced CO.sub.2 to bring said volume ratio into a desired high range. The catalytic carbon monoxide shift reaction is commonly conducted with a chromia promoted iron oxide catalyst at a flow rate of about 300-1000 standard cubic feet of gas per cubic foot of catalyst bed per hours, and at sufficiently elevated temperature to allow quasi-equilibration, which is usually about 700.degree. F.
Synthesis gas will undergo conversion to form reduction products of carbon monoxide, such as alcohols, at from about 300.degree. F. to about 850.degree. F., under from about 1 to 1000 atmospheres pressure, over a fairly wide variety of catalysts. The types of catalyst that induce conversion include ZnO, Fe, Co, Ni, Ru, ThO.sub.2, Rh and Os.
Catalysts based on ZnO are particularly suited for the production of methanol and dimethyl ether. Catalysts based on Fe, Co, and Ni, and especially Fe, are particularly suited for the production of oxygenated and hydrocarbon products that have at least one carbon-to-carbon bond in their structure. With the exception of ruthenium, all practical synthesis catalysts contain chemical and structural promoters. These promoters include copper, chromia, alumina and alkali. Alkali is of particular importance with iron catalysts, since it greatly enhances the conversion efficiency of the iron catalyst. Supports such as kieselguhr sometimes act beneficially.
The catalyzed reduction of carbon monoxide or carbon dioxide by hydrogen produces various oxygenated and hydrocarbon products, depending on the particular catalyst and reaction conditions chosen. The products that are formed include methanol, dimethyl ether, acetone, acetic acid, normal propyl alcohol, higher alcohols, methane, gaseous, liquid, and solid olefins and paraffins. It should be noted that this spectrum of products consists of aliphatic compounds; aromatic hydrocarbons either are totally absent or are formed in minor quantities.
In general, when operating at the lower end of the temperature range, i.e. from about 300.degree. F. to about 500.degree. F., in the reduction of carbon monoxide, and with pressures greater than about 20 atmospheres, thermodynamic considerations suggest that aliphatic hydrocarbons are likely to form in preference to their aromatic counterparts. Furthermore, in some catalytic systems it has been noted that aromatic hydrocarbon impurities in the synthesis gas inactivate the synthesis catalyst, and one may speculate that a number of known synthesis catalysts intrinsically are not capable of producing aromatic hydrocarbons.
The wide range of catalysts and catalyst modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide considerable flexibility toward obtaining selected products. Nonetheless, in spite of this flexibility, it has not proved possible to make such selections so as to produce liquid hydrocarbons in the gasoline boiling range which contain highly branched paraffins and substantial quantities of aromatic hydrocarbons, both of which are required for high quality gasoline. A review of the status of this art is given in "Carbon Monoxide-Hydrogen Reactions," Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, 2nd Edition, Volume 4, pp. 446-488, Interscience Publishers, New York, N.Y., the text of which is incorporated herein by reference.
Oxygenated compounds and hydrocarbons are produced in varying proportions in the conversion of synthesis gas. This is understandable if, as proposed by some researchers in the field, the hydrocarbons arise via oxygenated intermediates such as alcohols. By selection of less active catalysts such as zinc oxide, it is possible to obtain oxygenated compounds as the major product. One particular commercial conversion is used to produce methanol from synthesis gas with substantially no coproduction of hydrocarbons. Suitable catalysts are those comprising zinc oxide, in admixture with promoters. Copper or copper oxide may be included in the catalyst composition. Particularly suitable are oxide catalysts of the zinc-copper-alumina type. Compositions of the type described are those currently used in commercial methanol synthesis. Contact of the synthesis gas with the methaol synthesis catalyst is conducted under pressure of about 25 to 600 atmospheres, preferably about 50 to 400 atmospheres, and at a temperature of about 400.degree. F. to 750.degree. C. The preferred gas space velocity is within the range of about 1,000 to 50,000 volume hourly space velocity measured at standard temperature and pressure. It is noted that the conversion per pass is from about 10% of the carbon monoxide fed to about 30%, i.e. in this process the unconverted synthesis gas must be separated from the methanol product and recycled.
Crystalline aluminosilicate zeolites have been contacted with methanol under catalytic conversion conditions. U.S. Pat. No. 3,036,134 shows a 98.4% conversion of methanol to dimethyl ether over sodium X zeolite at 260.degree. C.; 1.6 mole % of the product is a mixture of olefins through pentene, with butene the predominant product. Conversion of methanol over rare earth exchanged and zinc exchanged X zeolite has been reported to produce some hexanes and lighter hydrocarbons (see Advances in Catalysis, Vol. 18, p. 309, Academic Press, New York, 1968). It has recently been discovered that alcohols, ethers, carbonyl and their analogous compounds may be converted to higher hydrocarbons, particularly high octane gasoline, by catalytic contact with a special type zeolite catalyst. This conversion is described in copending U.S. Patent Applications, Ser. Nos. 387,224, 387,223, and 387,222 filed on Aug. 9, 1973.
It is an object of the present invention to provide an improved method for converting fossil fuels to high quality gasoline. It is a further object of this invention to provide a method for converting a mixture of gaseous carbon oxides with hydrogen to high quality gasoline. It is a further object of this invention to provide a novel method of converting synthesis gas to high octane gasoline. It is a further object of this invention to provide a process for the manufacture of substantially sulfur-free liquid hydrocarbon fuels. Further objects of this invention will be apparent to those skilled in the art.